Circadian rhythms play a primary role in the adaptation of an organism to its environment. In mammals the visual system is necessary for entrainment of circadian rhythms to the 24 hour light-dark cycle but circadian rhythms are maintained with a period approximating 24 hours by endogenous central nervous system mechanisms in the absence of external cues. Evidence is available indicating that a retinohypothalamic tract, terminating in the suprachiasmatic (SCN) hypothalamic nucleus, is sufficient for entrainment of circadian rhythms to the light-dark cycle. Ablation of the SCN results in severe alteration, if not elimination, of all circadian rhythms thus far studied. The objective of this research program is to provide further evidence establishing the SCN as a CNS circadian oscillating system in mammals and to study the mechanisms of circadian rhythm generation. Specific studies will analyze the development of SCN using electron microscopy and the 2-deoxy-D-glucose method, both in vivo and in vitro. These will include an analysis of maternal influences on development of SCN circadian function. Other studies will analyze structure and circadian function in the partially or completely deafferented SCN. The overall goal of the program is to characterize the organization and properties of a circadian oscillating system in the mammalian brain.